1. Field of the Invention
This invention involves radio frequency (RF) and infrared frequency (IR) seeker systems. More particularly, this invention involves multiple RF and IR seeker means mounting on gimbals. The disclosed invention is suitable for use as a forward-mounted target emissions detector for a guided missile.
2. Description of the Related Art
Seeker systems using IR and RF detection means for guiding missiles to their targets are well-known in the art. Targets such as aircraft and cruise missiles have become increasingly non-cooperative to existing IR and RF seeker systems, whether active, semi-active or passive in design. Because any such seeker system is subject to exploitation of particular weaknesses by countermeasures, an effective guided missile must have several seeker means available for target acquisition and lock. This design strategy avoids the possibility of defeat by a single countermeasure.
In particular, virtually all missile seeker systems require IR seeker capability because of its high bearing accuracy. IR seeking means become more important as targets become more maneuverable and velocities increase, especially during the terminal phase of the encounter. But IR by itself is not sufficient for most long range incoming threats because (a) detection range is relatively short, (b) performance is not adequate in certain weather conditions, and (c) very effective countermeasures are available to a target that is aware of the attack. RF seeker systems have a long range all-weather capability but do not provide the tracking accuracy of the IR seeker system. Thus, for many battle scenarios, equipping a guided missile with a plurality of seeker modes can significantly increase the engagement kill probability over that of a single or dual seeker mode.
In the art, the commonly accepted method for designing a dual-spectral seeker system involves mounting the IR system in front of a RF antenna system at the forward end of the guided missile radome. The IR seeker means typically uses Cassegrainian optics and the RF seeker system typically uses a monopulse type antenna with the usual stringent directivity requirements of high gain and low sidelobes. The azimuth and elevation difference channel nulls and beam peak symmetry provided by the RF antenna system must give steering accuracy sufficient for the application. But merely placing an IR seeker system forward of an RF antenna seeker system will seriously degrade the RF antenna radiation parameters as is known in the art. These adverse effects include (a) reduced gain, (b) increased sidelobes, and (c) difference channel degradation in the form of null-filling and asymmetric beam peaks.
Because of this difficult problem, a number of schemes have arisen in the art which combine a single IR seeker with a single RF seeker system. Although long sought, coaxial integration of an IR seeker with two or more RF seeker systems without serious performance degradation to one or more such systems is presently unknown in the art.
U.S. Pat. No. 2,972,743 issued to Svenson et al. discloses a combined infrared-radar antenna system using a Cassegrainian reflector and a RF antenna which is transparent to infrared radiation. This combination is not suited for high performance IR or monopulse RF operation because of unavoidable degradation resulting from interference between the two antenna systems. Neither does this design allow for a second RF antenna system mounted behind the combined RF-IR assembly because both the IR reflector and the first RF antenna is opaque to RF radiation.
U.S. Pat. No. 3,165,749 issued to Cushner discloses a single reflector for use in both a Cassegrainian IR system and an RF detection system. The RF seeker performance of this dual detector is necessarily compromised by the IR detection performance requirements and the design does not allow addition of a second RF seeker mode. U.S. Pat. No. 3,701,158 issued to Johnson discloses a stripline-fed slotted array with the center removed to make room for an embedded IR seeker system. This dual seeker system suffers in performance because of the removed center section of the array which seriously perturbs the amplitude distribution of the RF seeker system, causing significant sidelobe levels.
U.S. Pat. No. 4,264,907 issued to Durand, Jr. et al. discloses a dual mode seeker system but does not teach the combination of the two systems into an integrated coaxial seeker assembly. A similar dual mode seeker system is disclosed in Japanese Pat. 62-879 by Yoshizawa. Neither system is intended for coaxial operation.
U.S. Pat. No. 4,282,527 issued to Winderman et al. and U.S. Pat. No. 4,652,885 issued to Saffold et al. both disclose a multi-spectral detection system which uses a Cassegrainian reflector for both IR and RF detection. In both patents, the detected IR and RF signals are separated by apparatus located behind the Cassegrainian reflector. Neither of these designs is suitable for addition of a second coaxial RF seeker system.
U.S. Pat. No. 4,477,814 issued to Brumbaugh et al. discloses a combined RF/IR system using a common surface for radiating and absorbing RF energy and reflecting and focusing IR energy. This design uses an RF-transparent Cassegrainian mirror placed in front of an RF slotted array antenna to permit use of the full aperture area for the RF and IR portions of the combined detection system. But this design does not permit the addition of a second coaxial RF seeker system because the first RF antenna is opaque to all RF radiation. For obvious reasons, the patent neither teaches nor suggests the use of a second coaxial RF detection system integrated with the disclosed dual mode RF/IR system.
The multi-spectral imaging system of Droessler et al. (U.S. Pat. No. 4,866,454) employs a sub-reflector surface that is transparent to millimeter-wave RF and reflective to IR radiation. The imaging system has a relatively long focal length (f/d=0 .55) and, as such, requires a large aperture for achieving the desired field of view. Since this area must be kept open, it would be necessary for the array antenna which is placed behind the antenna assembly to have an opening at the center of its aperture as well if an attempt were made to add an additional RF antenna by the use of a dichroic surface for the main reflector. This extra opening would result in a significant compromise in the antenna's gain and side-lobe level characteristics, thus making it undesirable to add a second RF antenna behind the antenna assembly.
As this brief discussion shows, those practicing in the art have not solved the specific problem of integrating multiple coaxial RF seeker antennas without causing performance degradation so severe that the expected increase in kill probability is lost. This is so although the problem has been long known in the art, moving many to search for a solution.